At present, normally the construction of underwater permanent building, such as sea entry road and artificial island, is to fill grit into water first until the height is higher than the water level, and then to construct slope protection dam, i.e. artificial island with slope protected by sand bag, artificial island with natural slope and artificial island at the expense of beach. By this method, the amount of grit used increases in geometric order with water depth. It costs much and has short life. Another method is to fill a square-shaped assembly with grit, solidified soil in bags, or concrete which does not spread in water and then to heighten it with cast-in-place concrete with mould plate after it is higher than the water level. Because square-shaped assemblies are repeated and there is no stake foundation, it is easy to be destroyed by stormy waves. The disadvantage of this method lies in: not only the seabed foundation needs to be disposed during construction, but also the hoisting of the assembly is too frequent when the assembly is small, or is inconvenient when the assembly is too big and heavy. Accordingly, this method normally takes a long construction period and high cost while safety factor is low.